Working for a foreign character, always being below expected grade (13)
I believe the answer is:
underachiever
'being below expected grade' is the definition.
'working for a foreign character always' is the wordplay.
'working for' becomes 'under' (I've seen this in other clues).
'foreign character' becomes 'chi' (Greek letter).
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'under'+'a'+'chi'+'ever'='UNDERACHIEVER'
(Other definitions for underachiever that I've seen before include "One doing less well than expected" , "Person not reaching their potential" .)